Apparatus for communication systems



27, 1938. W P, PLACE Y 2,131,388

APPARATUS FOR COMMUNICATION SYSTEMS HLS ATTORNEY sept 27, 193s. W. P. PLACE 2,131,388

APPARATUS FOR COMMUNICATION SYSTEMS Filed sept. 2o, 1955 2 sheets-sheet 2 6 fMegohms. l Szfgnalzh/ Current.

| l I I I T 5 2 1 o 1 2 5 0 d V0 lts Carrier Volts Hg, 5. fig. 4.

INVENTOR BWL/1...@

HIS ATTORN EY Patented Sept. 27, 1938 UNITED STATES APPARATUS FOR. COMMUNICATION SYSTEMS Willard P. Place, Wlkinsburg, Pa., assigner to Union Switch & Signal Company, Swissvale, Pa., a corporation of Pennsylvania Application September 20.,l 1935, Serial No. 41,444

11 Claims.

My invention relates to apparatus for communication systems, and particularly to apparatus for carrier telephone systems.

I will describe two forms of apparatus embodying my invention, and will then point out the novel features thereof in claims.

A feature of my invention is the provision of novel and improved apparatus for production and detection of a carrier signaling current, and w which apparatus is effective to maintain a balanced condition and in which the usual filter may be relatively simple. A further feature of my invention is the provision of' apparatus of the type here involved which is comparatively simple 15 and hence less costly to manufacture and easier to maintain. Other features and advantages of my invention will appear as the specification progresses.

Apparatus embodying my invention may be useful in many communication systems and is specifically adaptable to carrier telephone systems for railway trains. Inv such telephone systems for railway trains, it has been proposedV to utilize the trac rails as part of the communication channel, current being transmitted from and received at a station located on artrain through circuit elements electrically coupled with the traffic rails. In such systems two-way communication is accomplished on the same channel frequency and to that endl each station is provided with transmitting apparatus and receiving apparatus together with circuit switching devices operative to effectively couple either the transmitting apparatus or the receiving apparatus with the traic rails. Y

For a better understanding of my invention, reference may be had to the accompanying drawings wherein the invention is disclosed in connection with a railway train telephone system. It is to be understood, of course, that this specific applicationl is by way of illustration only, and apparatus embodying my invention is equally useful for other communication systems.

In the accompanying drawings, Fig. 1 is a 45 diagrammatic view ofl one form of apparatus embodying my invention applied to the train carried equipment of a telephone system for railway trains. Fig. 2 isa diagrammatic View of a modified form ofthe balanced' frequency trans- 50 lating unit of' Fig. 1,Y and which also embodies myinvention. Figs; 3 and-4 are diagrams illustrating characteristic curves of the frequency translating units of Figs. 1 and 2.

In each of the different views, like reference characters refer tolike parts.

able sources of current and switching devices. ,m

As here shown, the telephone set TS'includes Va loud speaker LS and a microphone M, the microphone being preferably mounted on ahandle 5 to facilitate its` being broughtrclose tothe person speaking. Associated with. the microphone han- 15 dle 5 is a switching device including two push buttons 2 and 3 which are of the spring return type, the push button 2 closing a contact 2-6 when depressed and the push button 3v closing a Contact 3 1 when depressed; The .1311511 button 3 controls a directional relay DR over a simple circuit including the B terminal of any convenient source of current such-as a battery not shown, winding of relay DR, wire 4,.contact 3--'l,' and to the opposite terminal C of the same, source of current. The pushbuttonZ is effective to'con.- trol a calling relay CR as well as the directional relay DR overY av circuit'extending from the '.B terminal ofthe current source, winding of relay DR, wire 4co`ntact 2 6., wire 8winding of relay CR, and to the terminalV C ofthe current source. It follows that both relays DR and CR are normally deenergized, relay DR is. picked Vup. when the push button 3 is depressed, and` both relays'Y DR and CR are Vpicked up whenthe push button 2 is. depressed. The microphone Mfis interposed in a local circuit includinga battery 40', a front contact 4| ofthe directional relay DR, primary Winding 42 of a transformer T3 to`be referredto later, a second front contact .43 of directional 40 relay DR, microphone M, `andto theopposte terminal of the Vbattery 4U. .The loud speaker LS is provided with an operating circuit which includes a secondary winding 44- of an output transformer T4 the primary winding 45 of which is interposed 4,5v

in the plate circuit ofthe electron tube of the low` frequency amplifier LA as-will bepointedout later. Hence, audio frequencies supplied tothe input of the amplifier LAin a manner. to later appear are reproduced at theloud-speaker.. The 59 further functions of the directional-relay DR and the calling relay CR willappear when the operation of the apparatus of Fig. 1 is described.

The receiving apparatusR and the transmitting apparatus T mayeach takediferent forms 5f? several of which are well known to the art, and they will be described in this description only in so far as is necessary for a full understanding of my invention. As here shown, the receiving apparatus R includes a band pass filter BPF and two resistance coupled stages of amplification. The tubesl and Il of the respective stages of amplification are preferably of the heater screen grid type, the filaments of each tube being normally heated in the usual manner. 'Ihe plate circuit for the tube I0 is supplied with current from a generator G driven from any convenient source of power such as the usual headlight generator of the locomotive L. This plate circuit extends from the positive terminal of thegenerator G over wire I2, back contact I 3 of directional relay DR, reactor I4, resistors I5, I, and I l', plate I8 of tube I0, intervening tube Vspace to cathode I9, biasing unit 20, ground electrode 2| and thence over ground electrode 22 to the negative terminal of the generator G. The plate circuit for tube I I includes the following elements, positive terminal of generator G, wires I2 and 23, resistor 24, primary winding 25 of a transformer TI to be referred to later, plate 26 of tube II, intervening tube space to cathode 21, biasing unit 28 and ground electrodes 29 and 22 backto the generator G. It follows that the first stage tube Ill is supplied with plate voltage only when directional relay DR is deenergized and its back contact I3 is closed, but that the second stage tube II is constantly supplied with plate voltage. Y

The filter BPF is adapted to pass a side band telephone current say, for example, the upper side band of a carrier current of 8000 cycles modulated with voice frequencies, and to substantially suppress frequencies both above and below such side band. The input side of the lter BPF,1 is connected over wires 46 and 41 with an inductor 9 mounted on the locomotive L in inductive relation with the traffic rails. 'Ihe output side of the filter BPF is connected over wires 48 and 49 with the control grid circuit of the first stage tube I0, the output of which tube is coupled with the control grid 50 of the second stage tube I I in the usual manner. Consequently, as long as the directional relay DR is deenergized and plate voltage is applied to the' rst stage tube Ill as well as to the second stage tube II, electromotive forces induced in the inductor 9 in response to an upper side band carrier telephone current of the frequency here assumed flowing in the traiiic rails are filtered, amplified, and reproduced in the primarywinding 25 of the transformer TI, but energization of the directional relay DR and opening its back contact .I3 renders the tube I0 without plate voltage and the receiving apparatus R is not responsive to such current flowing in the rails. llt will be understood, of course, that the receiving apparatus R may include additional stages of amplification should it seem desirable to do so.

The transmitting apparatus T includes an amplifier TF of one or more stages of amplification and an amplifying transmitter AT which preferably consists of power tubes that are capable of amplifying the outgoing current to a relatively high energy level. The output of the transmitcluding the front contact 32 of directional relay DR and ground electrodes 69 and 22 as will be understood by an inspection of Fig. 1. That is, the plate circuits for the tubes of the amplifier TF are supplied with current from the generator G only when the directional relay is picked up, and hence the transmitting apparatus T is normally inactive and is rendered active by energizing the directional relay DR and closing its front contact 32.

The frequency translating unit FTU consists of an input transformer TI, an output transformer T2, a source of carrier frequency current 31, two copper-oxide rectifiers 33 and 34, and two filter circuit networks 35 and 36. The source of carrier frequency current 31 may be anyone of several forms among them being an electron tube oscillator. The frequency of the carrier current supplied by oscillator 31 would be 8000 cycles per second since it has already been assumed that the carrier telephone current is the upper side band of a carrier of 8000 cycles modulated with voice frequencies. It will be understood, of course, that other carrier frequencies may be selected if desired. The opposite terminals of the oscillator 31 are connected between the mid terminal 5I of the secondary winding 38 of input transformer TI and the mid terminal 52 of the primary winding 39 of output transformer T2, the parts being so proportioned that the two windings 38 and 39 are preferably alike in construction and the two portions of each winding on the opposite sides of its mid terminal are substantially identical. The rectifier 33 is connected between the top outside terminals of windings 38 and 39 and the rectifier 34 is connected between the lower outside terminals of these two windings, and both rectifiers are disposed with their forward direction toward the primary winding 39. Furthermore, the rectifiers 33 and 34 are matched, that is, they possess substantially identical characteristics.

The impedance offered by each of the copperoxide rectifiers 33 and 34 is illustrated in Fig. 3 which is a typical voltage-impedance characteristic curve for such rectifiers. In response to carrier voltage from oscillator 31 applied between terminals 5I and 52, current ofthe carrier frequency flows from the mid terminal 5I through Vthe balanced secondary winding 38 of the input f transformer, the two copper-oxide rectifiers 33 and 34, the balanced primary winding 39 of the output transformer and returns to the opposite terminal of the carrier source from Vthe mid terminal 52. current flowing through a copper-oxide rectifier in the forward direction due to the voltage applied thereto lowers the impedance of the rectifier, and a current flowing through the rectifier in the high resistance direction increases the impedance up to a certain limit, the impedance of eachY rectifier 33 and 34 is changed in accordance with theinstantaneous value of the carrier frequency current. Consequently, since the transfer of energy from the balanced input transformer TI to the balanced output transformer T2 is controlled by the impedance offered by the rectifiers 33 and 34, this transfer of energy is Varied in accordance with the instantaneous value of the carrier frequency Voltage of oscillator 31.

Signaling current, which in this instance is a telephone current, produced in the primary wind-l ing 25 of transformer TI induces electromotive forces of corresponding frequencies in the secondary win-ding 38 of that transformer. Such Since, as illustrated in Fig. 3, a

,quency Voltage.

electromotive forces induced in secondary winding 38 produceV currentswhich ow in the bailanced circuit including the rectifiers 33 and 34 and the primary winding 39 of transformer T2. These currents thus Vproduced by the incoming electromotive forces of the signaling current are relatively small in comparison with the current supplied by the oscillator 31 and their effect .on the impedances of the rectifiers 33 and 34 is negligible. Since the impedances .of the .rectiers 33 and 34 .vary with the instantaneous value of the carrier frequency voltage, the signaling current passed in response to the signaling electromotive force is varied, that is, the signaling energy transferred from the input transformer TI to the output transformer T2 is varied with the instantaneous values of the carrier fre- Variation in the signaling current passed in response to the incoming electromotive forces is illustrate-d in Fig. 4, the plus and minus values of the carrier Voltage being indicated by the axis of ordinates and the relative value of the signaling current passed being` indicated by the axis of abscissas, In other words, at the instant the carrier voltage is zero, the signaling current passed by the rectifiers in response to a given value of electromotive force induced in the secondary winding 38 of the input transformer is represented by the point X of Fig. 4. For positive values of the carrier voltage the signaling current passed in response to the same given value of electromotive force is indicated by the right-hand portion of the curve of Fig. 4. For negative values of the carrier volt age, the current passed in response to said given Value of electromotive force is represented by the left-hand portion of the curve. Hence, the effect of the copper-oxide rectifiers is to pro-duce a heterodyning action of the carrier and the incoming signaling frequencies with the result that the electromotive forces induced in the secondary windings 53 and 5.4 of the output transformer T2 contain upper and lower side band frequencies of the carrier and signaling currents, suppression of the carrier being accomplished by the balanced circuit.

The filter circuit network 3.6 which is connected to the secondary winding 54 of transformer T2 is so proportioned and adjusted as to pass vthe upper side band frequencies of the carrier modulated with Voice frequencies, and the lter circuit network 35 which is connected to the secondary winding 53 is so proportioned and adjusted as to pass the lower side band resulting from an upper side band telephone current'rnixed with the carrier frequency of oscillator 31. That is, the network 35 passes the audio frequencies resulting from demo-dulation of an upper side band telephone current, and the filter 35 passes the llDlDer side band frequencies resulting from mod ulation of the carrier with voice frequencies. The output side of the filter network 35 is con.- nected over wires 51 and 58 with the input of the low frequency amplifier LA, the output circuit of which includes the primary winding of transformer T4. The amplifier LA may be of any standard type and may consist of one or more stages of amplification as desired. Plate voltage for the tube of amplifier LA is obtained from the generator G over a circuit including wire I2, back contact I3 of directional relay DR, reactor I4, resistor I5, wire 55, primary winding 45, plate-cathode of the tube of amplifier LA, and ground electrodes 56 and 22 back to the generator G. It follows that as long as the directional relay DR is deenergized and" its back contact .I3 is closed, audio frequencies passed by the filter 35 are amplified at LA and reproduced by the loud speaker LS, but 4that the amplifier LA is rendered inactive in response to energizing the directional relay. The output side of the filter 3 6 is connected over wires 59 and .Sil with the input of the amplifier TF of the transmitting apparatusl T. Consequently upper side band frequencies passed by the filter 36 are supplied to the 'transmitting apparatus where they are amplified and supplied to the traffic rails, the transmitting apparatus being active however only when the directional relay DR lis' picked up and its front contact 32 is closed as pointed out hereinbefore.

Voice frequencies are supplied to .the input transformer v'II of the frequency translating unit FTU for modulation of the carrier in the following manner: The microphone M is connected with the primary winding 42 of transformer T3 over front contacts 4I and 43 of the directional in the potential of the screen grid 63 of tube II with respect to the cathode of that tube. The

tube II then moderately amplifies in its plate circuit these variations of the Voltage applied to its screen grid 5 3 and electromotive forces are induced in the secon-dary winding 38 of transformer TI which correspond to the voice frequencies produced in the microphone M, these electromotive forces being Aeffective to modulate the carrier in the manner already described.

In describing the operation of the apparatusY of Fig. l, I shall first assume the equipment to be in the position illustratedand the receiving apparatus R and the low frequency amplifier LA both supplied with plate voltage.Y Under these conditions, electromotive forces induced in the inductor 9 due to a side bandtelephone current flowing in the traffic rails are filtered and amplified by the apparatus R and reproduced in the secondary winding 38 of the input transformer of thefrequency translating unit FI'U. Here these upper side band frequencies are mixed with the carrier and demodulated and the resultant audio vfrequencies are passed by the filter network 35 to the amplifier LA where they are further amplified and then reproduced at the loud speaker LS. The energy consumed bythe filter network 3.5 will be negligible due to the tuning of the network and due to the fact that the transmitting apparatus is now inactive. To send a ytelephone current, the operator will pick up the handle '5 andy depress the push button 3 to energize the directional re lay DR and transfer plate voltage from the tubes of the receiving apparatus to the tubes of the transmitting apparatus except for the tube II which is constantly supplied with voltage. Voice frequencies now produced in the microphone M are applied to the screen grid 63 of tubel II and amplified and reproduced in the secondary winding 38 of transformer TI where they modulate the carrier. The upper side band frequencies resulting from such modulation are passed by the filter network 36 to the transmitting apparatus T and are supplied to the traffic rails at a relatively high energy level. The receiving apparatus R will not respondto such current since the tube l is now without plate voltage, and any energy passed by the filter 35 will not be reproduced at the loud speaker LS sincethe amplifier LA is now without plate voltage.

In such carrier telephone systems it has been proposed to provide a calling current which is equivalent to the carrier modulated by a single predetermined voice frequency. This callingcurrent may be produced by shifting the frequency generated by the local carrier oscillator. Such shifting of the carrier frequency may be accomplished by changing the inductance or capacitance of the oscillating circuit associated with an electron tube oscillator. In Fig. 1, shifting of the carrier frequency is accomplished by connecting a part of the high pass lter with the oscillator over contacts of the calling relay CR. When the operator desires to send a calling current, he depresses the push button 2 and energizes both relays DR and CR. The picking up of the directional relay DR transfers plate voltage from the receiving tubes to the transmitting tubes the same as explained heretofore.. The picking up of the calling relay CR connects the oscillator 31 across the last inductance element 64 of the filter 36 over front contacts 65 and 66. The parts are so proportioned and adjusted that when the inductor S4 is interposed in the circuit of the oscillator 31 the carrier frequency is shifted, say '700 cycles, that is, the frequency supplied by the oscillator 31 is now 8700 cycles per second. The voltage thus created across the inductor 64 is impressed over wires 59 and 60 on the input of the transmitting apparatus T and consequently a calling current of 8700 cycles per second is supplied to the traffic rails in response to picking up the calling relay CR.

In the modification of the frequency translating unit FTU disclosed in Fig. 2, a resistor 61 and a biasing battery 68 are interposed between the mid terminals 5l and 52 and the oscillator 31 is connected across the resistor 61. The bias effected by the battery 68 provides operation of the copper-oxide rectifiers at the best part of their characteristics. The resistor S1 should be of relatively low resistance so that the change in impedance of the rectiers will not cause any appreciable distortion of the impressed voltage of the carrier frequency. It is clear that the frequency translating unit FTU when constructed in the manner disclosed in Fig. 2 and applied to the telephone equipment of Fig. 1 will function in the production and detection of carrier telephone current in a manner similar to the unit of Fig. 1 and it is thought to be unnecessaryrto repeat the operation in detail.

Telephone equipment constructed in accordance with my invention is relatively simple and of small power consumption. The frequency translating units constructed in the manner here described effect a balanced circuit condition by which suppression of the carrier is accomplished with but small unbalance even if the two copperoxide rectiers are not identical in characteristics. In practicing my invention, the copper-0X- ide rectiers would have a resistance in their non-pass direction which is at least one hundred times their resistance in the pass direction and the circuit following the copper-oxide rectifiers would preferably be designed with an impedance of approximately ten times the impedance of the copper-oxide elements during that part of the cycle of the local carrier which vcauses the impedance to be low and approximately one-tenth as high as the impedance of the copper-oxide elements during the time when the local carrier causes the impedance to be high. During onehalf cycle of the local carrier 90 per cent, of the impressed voltage will pass through the translating unit, and during the other one-half cycle per cent. of the impressed voltage will be transferred. With such proportioning of the parts and a 10 per cent. difference in the characteristics of the copper-oxide elements occurs, then during one half cycle one rectifier, say rectifier 33 of Fig. 1, is 10 per cent. of the load impedance and the other rectifier 34, 9 per cent. of the load impedance. Thus a 10 per cent. unbalance of a'rectier element represents but 1 per cent. difference in the total impedance and as a consequence only 1 per cent. of the carrier current is left unbalanced with the result that the filter circuit network following the translating unit `may be relatively Vsimple in construction.

As here disclosed and described, a single frequency translating unit is provided and this one unit functions for both sending and receiving. It is clear, however, that two such frequency translating units may be provided for the telephone equipment, one to be connected with the receiving Aapparatus for demodulation of the incoming telephone current and the other connected with the transmitting apparatus for modulating the outgoing telephone current. Again, in the case two translating units are provided. a single source of carrier frequency may supply current to the two units in multiple.

Although I have herein shown and described only two forms of apparatus embodying my invention, it is understood that various changes and modifications may be made therein within the scope of the appended claims without departing from the spirit and scope of my invention.

Having thus described my invention, what I claim is:

1. In a communication system comprising a communication circuit and receiving apparatus effectively influenced by a modulated carrier frequency current coupled with said circuit; the combination with the above instrumentalities which consists of a frequency translating unit including an input transformer, an output transformer, a source of carrier frequency current connected between a mid terminal of the input transformer secondary winding and a mid terminal of the output transformer primary winding, and two copper-oxide rectiers one connected between two outside terminals of said windings and the other connected between the other two outside terminals of said windings; a communication device responsive tothe modulation frequency of said modulated carrier current, means to effectively connect the output of said receiving apparatus with the primary winding of said input transformer, and means including a low-pass filter to connect the secondary winding of said output transformer with said communicating device.

2. In a communication system comprising a communication circuit, transmitting apparatus adapted to transmit a carrier current modulated by a voice frequency current, receiving apparatus effectively infiuenced by such modulated carrier current and switching means for effectively coupling either the transmitting or the receiving 'apparatus with the circuit; the combination with the above instrumentalities which consists of a frequency translating unit including an `input transformer, an output transformer, a source of carrier frequency current connected between a mid terminal of the input transformer secondary winding and a mid terminal of the output transformer primary winding, and two copper-oxide rectiers one connected between two outside terminals of said windings and the other connected between the other ktwo outside terminals of said windings; a source of Voice frequency current, a telephone device responsive to such voice frequency current, means to effectively connect either the output of said receiving apparatus or said source of voice frequency current with the primary winding of said input transformer, a high pass filter to connect a secondary winding of said output transformer with the input of said transmitting apparatus, and a low pass lter to connect another secondary winding of the output transformer with said telephone device.

3. In a communication system comprising a communication circuit, transmitting apparatus adapted to transmit a carrier telephone current, receiving apparatus effectively influenced by such telephone current, and switching means for effectively coupling either the transmitting or the receiving apparatus with the circuit; the combination with the above instruinentalities which consists of a frequency translating unit including an input transformer, an output transformer, a source of carrier frequency current connected between a mid terminal of the input transformer secondary winding and a mid terminal of the output transformer primary winding and two copper-oxide rectifiers one connected between two outside terminals of said windings and the other connected between the other two outside terminals of said windings; a microphone, a telephone receiver, a directional relay, means controlled by the directional relay to effectively connect either the output of said receiving apparatus or said microphone with the primary winding of said input transformer, other means controlled by the directional relay to effectively connect the secondary Winding of said output transformer with either the input of said transmitting apparatus or said telephone receiver, and manually operable means to govern said directional relay.

4. In a communication system comprising a communication circuit, transmitting apparatus adapted to transmit a carrier current modulated by a voice frequency current, receiving apparatus effectively influenced by such modulated carrier current and switching means for effectively coupling either the transmitting or the receiving apparatus with the circuit; the combination with the above instrumentalities which consists of a frequency translating unit including an input transformer, an output transformer, a source of carrier frequency current and two copperoxide rectiers; means including a biasing battery and a resistor to connect a mid terminal of the input transformer secondary winding with a mid terminal of the output transformer primary winding, means to connect one of the rectifiers with two outside terminals of said windings and to connect the other rectifier with the other two outside terminals of the windings,

means to connect said carrier source across said resistor, a source of voice frequency current, a telephone device responsive to such voice frequency current, means to effectively connect either the output of said receiving apparatus or said Vsource-of .voice 'frequency current with the primary winding of said linput transformers, a high -pass filter :and a low `pass filter, and means to eifectivelyconnect the secondary winding of the output transformer with the vtransmitting apparatus vthrough said high pass filter or to connect said winding :with the telephone device through said 'low -pass filter.

5. A communication system comprising, acommunioation circuit, receiving apparatus including an electron -tube amplifier having a control grid and a screen grid; a frequency translating unit consisting of'an input transformer, an output transformer, a source of carrier current, two copper-oxide rectiers,a low pass filter and a high pass filter; said'carrier source and rectiers interposed in circuit connections between the secondary winding of the input transformer and the primary winding of the output transformer Vto-V heterodyne a telephone current transferred between said windings, Va microphone, a loud speaker, means to connect the plate of said tube with the primary winding of said input transformer, a first secondary winding of the output transformer connected to the low pass filter, a second secondary `winding of the output transformer connected to the high pass filter, a first circuit means to effectively couple either the communication circuit to the controlgrid ofsaid tube or to couple said microphone to the screen grid of the tube, a second circuit means to effectively couple either the high pass filter to the communication circuit or the low pass lter to the loud speaker, and a manually operated switching device to control said first and second circuit means. Y

6. In combination, yan .input transformer, an output transformer, a source of vcarrier frequency current, means to connect said source between a mid terminal of the input itransformersecondary winding and a mid terminal of the output transformer primary winding, two copper-oxide rectifiers one connected between two outside terminals of said windings and the other connected between the other two outside terminals of said windings and said rectiers disposed with their forward direction toward said primary winding,

means including a high pass lter to connect the secondary winding of the output transformer with a transmitting circuit, and means including a microphone connected to the primary winding of the input transformer whereby an upper side band telephone current is supplied to said transmitting circuit.

'7. In combination, an input transformer, an output transformer, a source of carrier frequency current, means to connect said source between a mid terminal of the input transformer secondary winding and a mid terminal of the output transformer primary winding, two copper-oxide rectiers one connected between two outside terminals of said windings and the other connected between the other two outside terminals of said windings and said rectiers disposed with their forward direction toward said primary winding, and means to supply a telephone current to the primary winding of the input transformer for producing side band frequencies of the carrier and said telephone current in the secondary winding of the output transformer with the carrier frequency suppressed.

8. In combination, an input transformer, an output transformer, a source of carrier frequency current, means to connect said source between a mid terminal of the input transformer secondary winding and a mid terminal of the output transformer primary winding, two asymmetrical units one connected between two outside terminals of said windings and the other connected between the other two outside terminals of said windings and said units disposed with their forward direction toward said primary winding, said units characterized by impedances which decrease in proportion to the current passed in their forward direction and which increase in proportion to the current passed in their backward direction, and means to supply a telephone current to the primary winding of the input transformer for producing side band frequencies of the carrier and said telephone current in the secondary winding of the output transformer with the carrier frequency suppressed.

9. A communication system comprising, a communication circuit, receiving apparatus including a first and a second stage electron tube amplifier, said second stage electron tube including a screen grid as well as a control grid; a frequency translating unit including a source of carrier current, a balanced circuit, a high pass filter and a low pass filter; said unit operative to effect modulation of the carrier with voice frequencies or to effect demodulation of a carrier telephone current, a microphone, a loud speaker, means to couple the plate and cathode of the second stage tube to the input terminals of theunit, means to couple the grid and cathode of the first stage tube with the communication circuit, a transmitting amplifier effectively coupled with said circuit, a source of plate voltage, a relay, means including a back contact of the relay to connect the source of plate Voltage to the plate of said first stage tube and to couple the loud speaker to said low pass filter, means including front contacts of the relay to effectively couple the high pass filter with the transmitting amplifier and to couple the microphone to the screen grid of said second stage tube, and means to control said relay.

10. A communication system comprising, a communication circuit, receiving apparatus including a first and a second stage electron tube amplifier, a source of plate voltage for said tubes, a demodulator operative to demodulate a carrier telephone current, a loud speaker coupled to the output terminals of the demodulator, means to permanently connect the plate and cathode of the second stage tube to the input terminals of the demodulator, means to permanently connect the grid of the second stage tube to the plate of the first stage tube and to permanently couple the grid of the first stage tube to said circuit, and means to at times disconnect the plate Voltage source from the plate of said rst stage tube whereby said second stage tube and demodulator are uninfiuenced by current flowing in the communication circuit.

11. A communication system comprising a communication circuit; a frequency translating unit including a source of carrier curr-ent of a given frequency, a balanced circuit and a high pass filter consisting of capacitance and inductance; said unit effective to modulate the carrier with voice frequencies with the carrier suppressed and to select the upper side band, means to couple the final inductance of the filter with said circuit, means including a microphone connected with the input terminals of said unit for supplying to the circuit an upper side band carrier telephone current, a calling relay, means controlled by said relay to associate the final inductance with said source of carrier current for supplying to said circuit a calling current of a frequency equal to said given frequency modulated at a predetermined voice frequency, and means to control said relay.

WILLARD P. PLACE. 

